The recent trend of electronic equipment is characterized by high speeds of size and weight reduction. Under the circumstances, it is desired to reduce the size and weight of semiconductor devices serving as the brain of electronic equipment. The ultimate target is to reduce the packaging area to the chip size, realizing a chip size package (CSP).
Epoxy resins are widely used in constructing semiconductor packages because of their good adhesion, heat resistance and moisture resistance. As the package system is diversified as mentioned above, epoxy resins are now used not only as the encapsulants well known in the art, but also as coating, die bonding and under-filling materials. Most such materials are diluted with solvents or in paste form.
Of these, the solvent-diluted materials require an attention to carefully remove the solvent that volatilizes off on use, from the standpoints of the health maintenance and safety of workers, undesirably adding to the cost.
Coating materials are used in several ways. One proposed method is to cover the surface of a semiconductor chip solely with a thin film of an epoxy resin composition to effect sealing. It is also proposed to screen print an epoxy resin composition in paste form. These materials include paste-like epoxy resin compositions such as those of the acid anhydride curing type and the amine curing type which are well known in the art. Alternatively, these paste-like epoxy resin compositions may take the form of a film which has been converted to B-stage.
Most of the prior art die bonding and under-filling materials were paste-like epoxy resin compositions. Recently, films formed from paste-like die bonding materials are widely utilized. On use, a film is pressed against a lead frame or substrate, and a semiconductor chip is joined onto the film whereupon the film is cured. Most of these die bonding materials are epoxy resin compositions of the acid anhydride curing type and the amine curing type which are well known in the art.
These B-staged films are easier to handle than paste-like materials and provide a very promising system that contributes to the simplification of a semiconductor device assembling process. However, the epoxy resin compositions of the acid anhydride curing type and the amine curing type are insufficient in the storage of uncured compositions and the moisture resistance and high-temperature performance of cured compositions. They are unsatisfactory in directly covering the semiconductor chip surface with a thin film.
As compared with the epoxy resin compositions of the acid anhydride curing type and the amine curing type, epoxy resin compositions of the self-polymerization type, especially those using imidazoles as the curing catalyst, and epoxy resin compositions of the phenolic curing type are satisfactory in the storage of uncured compositions and the moisture resistance and high-temperature performance of cured compositions, but are difficult to control the progress of reaction and hence, to form B-staged (or semi-cured state) films. To obtain flexible, easy-to-work films, the softening point of an epoxy resin base and a phenolic resin curing agent must be lowered. Undesirably, this is done at the expense of the heat resistance of cured compositions.
An object of the invention is to provide an epoxy resin composition of the self-polymerization type or the phenolic curing type which has a low glass transition temperature in an uncured state while maintaining the heat resistance, moisture resistance and low stress of a cured composition. Another object of the invention is to provide a laminate film comprising a flexible, easy-to-work thin layer of the epoxy resin composition. A further object of the invention is to provide a semiconductor device in which the gap between a semiconductor chip and a substrate or the surface of a semiconductor chip is sealed with the laminate film.